1. Field of the Invention
The present invention relates to a semiconductor device using a semiconductor element (an element that is formed of a semiconductor film), particularly, to a liquid crystal display. The present invention also relates to electronic equipment employing the liquid crystal display for a display unit.
2. Description of Related Art
A technique that has been attracting attention in recent years is to fabricate a thin film transistor (TFT) using a semiconductor film (with a thickness on the order of several nm to several hundreds nm) that is formed on a substrate having an insulating surface. The thin film transistor is applied to a wide range of electronic devices, including ICs and semiconductor devices. Its application to a switching element for a liquid crystal display is especially desired to be developed swiftly.
An active matrix liquid crystal display has a pixel portion comprising a plurality of pixels, each of which has a TFT (pixel TFT) and a liquid crystal cell. The liquid crystal cell has a pixel electrode, an opposite electrode, and a liquid crystal interposed between the pixel electrode and the opposite electrode. The pixel TFT controls the voltage applied to the pixel electrode, whereby an image is displayed in the pixel portion.
A TFT whose active layer is formed of a semiconductor film having a crystal structure (a crystalline TFT) has high mobility. Therefore the crystalline TFT makes it possible to integrate functional circuits on the same substrate so that the liquid crystal display thus constructed can display an image with high definition.
In this specification, the semiconductor film having a crystal structure includes a single crystal semiconductor, a polycrystalline semiconductor, a microcrystalline semiconductor, and semiconductors disclosed in Japanese Patent Application Laid-open Nos. Hei 7-130652, Hei 8-78329, Hei 10-135468 and Hei 10-135469.
In fabricating the active matrix liquid crystal display, a million to two million crystalline TFTs are needed in the pixel portion alone. In addition, the functional circuits to be provided in the periphery of the pixel portion also comprises crystalline TFTs to increase the total number of required crystalline TFTs. The liquid crystal display has to meet the strict standard and the reliability of each crystalline TFT has to be solid in order to achieve a stable image display.
Characteristics of the TFT can be divided into characteristics in ON state and characteristics in OFF state. The characteristics in ON state include ON current. mobility, S value and threshold. Of the characteristics in OFF state, OFF current is an important characteristic.
The liquid crystal display employing the thin film transistor (TFT) is frequently used as a light bulb in liquid crystal projectors and the like.
Projected light in projectors generally has an intensity of about million lux. Most parts of projected light irradiate the pixel electrode whereas a part of the projected light enters the active layer of the TFT provided on an active matrix substrate. When the projected light enters a channel formation region of the active layer, in particular, a photoelectric current is generated in the region due to photoelectric effect to thereby increase OFF current of the TFT undesirably.
It is thus indispensable to set a shielding film capable of shielding against light (black matrix) so as to prevent external light from entering the active layer of the TFT. Generally, the shielding film is provided on an opposite substrate or on the active matrix substrate.
In the case of providing the shielding film on the opposite substrate, however, the bonding technique currently used takes too large margin for positioning of the shielding film on the opposite substrate and the aperture ratio is lowered as a result. Therefore this arrangement of the shielding film might not remain suitable considering the expected trend toward miniaturization of semiconductor elements.
On the other hand, in the case of providing the shielding film on the active matrix substrate, the shielding film is formed usually above a transistor and a wiring that do not require to transmit visible light, with an interlayer insulating film interposed between the shielding film and the transistor and the wiring. This structure takes only a limited margin for positioning of the shielding film, and therefore can improve the aperture ratio.
Still, light can enter the active layer of the TFT from the active matrix substrate side if there is reflection light reflected at the surface of the active matrix substrate when the projected light passes through the liquid crystal display, or if plural liquid crystal displays are used in order to obtain color display and there is light that has passed through the other liquid crystal displays. In those cases, the shielding film of the above configuration has difficulties in suppressing OFF current of the TFT.